Put the currently focused thing in the URL fragment.

[zenhack]

This would enable:

• Linking directly to a definition
• Easily turning bits of pretty-printed code into links to other stuff in the codebase; we potentially do this for all identifiers.

[mitchellwrosen]

I think @exw's work on single-page-appifying the browser may cover this. Linking directly to a definition (or branch, or whatever) would be 👌

[exw]

Coming soon, there's a wip draft pr at #20 -- just wanted to add support for linking to terms and types before submitting a PR to master